In many electronic circuit applications, storage devices are needed primarily for reading data stored therein. Such memories, termed read only memories (ROMs), are initially programmed with the desired data, and are rarely reprogrammed thereafter. Commercial 16K and 64K ROM memories are currently available for storing such type of static data.
When the reprogramming of ROM memories is necessary, various techniques are available for writing new data therein. In certain types of ROM memories, the old data can be erased by ultraviolet light, while in more recently developed MOS memories the cells can be quickly electrically erased and reprogrammed. MOS read only memories of the latter type employ a floating gate and an electron tunneling affect for achieving multiple write operations and are termed electrically erasable programmable read only memories (EEPROMS).
While the floating gate type of storage cell greatly facilitates reprogramming, the cell structure deteriorates after a number of programming cycles, thereby reducing the reliability of the memory. Specifically, a very thin oxide layer separates the floating gate of the memory cell transistor from a diffused area in the substrate. After a number of programming or write cycles, a charge accumulates in the tunnel oxide, thereby reducing the effect of the programming voltage on the cell. As a result, the electric field across the oxide is reduced, and the charge transferred to the floating gate is correspondingly reduced. Hence, the programmability of the ROM memory is affected, and the reliability is reduced.
From the foregoing, it may be seem that a need has arisen for an EEPROM memory which can be revitalized after a number of programming write operations. There is a concomitant need for an EEPROM memory in which cell deterioration can be detected, and compensated for without the loss of data information.